1. Field of the Invention
The present invention relates to an organic electroluminescence (hereinafter simply referred to as “EL”) pixel circuit, which controls a drive current to be supplied to an organic EL element based on a data signal.
2. Description of the Related Art
Because electroluminescence display devices wherein an EL element which is a self-emitting element is used as an emissive element in each pixel have advantages such as the fact that the device is self-emitting, that the thickness can be reduced, and that the power consumption is small, they have attracted much attention as display devices offering alternatives to display devices such as liquid crystal display (LCD) devices and CRT (cathode ray tube) display devices.
In particular, an active matrix EL display device in which a switching element such as a thin film transistor (hereinafter simply referred to as “TFT”) for individually controlling an EL element is provided in each pixel and the EL element is controlled for each pixel can achieve a high resolution display.
In an active matrix EL display device, a plurality of gate lines extend along a row (horizontal) direction on a substrate, a plurality of data lines and power supply lines extend along a column (vertical) direction, and each pixel comprises an organic EL element, a selection TFT, a driver TFT, and a storage capacitor. When a gate line is selected, the selection TFT is switched on and a data voltage on the data line (voltage video signal) is charged into the storage capacitor. The driver TFT is switched on by the charged voltage and power is supplied from the power supply line to the organic EL element.
In this pixel circuit, when the threshold voltages of the driver TFTs in the pixel circuits arranged in a matrix form vary, the brightness varies, and as a result, there is a problem that the display quality is degraded. However, the characteristics of the TFTs in the pixel circuits over the entire display panel cannot be made identical to each other, and it is therefore difficult to prevent variation in the threshold values for switching the TFTs on and off.
It is therefore desirable to prevent the effects of variation in the threshold values among the driver TFTs on the display.
Various structures have been proposed as a circuit for preventing the effects of variation in the threshold values among the TFTs (for example, Japanese Publication of Unexamined PCT Patent Application No. 2002-514320).
These configurations, however, require a circuit for compensating for the variation in threshold value. Therefore, when these circuits are used, the number of elements in the pixel circuit is increased, resulting in a problem that the aperture ratio is reduced. In addition, when a circuit for compensation is added, there is a problem that the peripheral circuit for driving the pixel circuit must also be changed.